1. Field of the Invention
The present invention relates to a data round-off device for use in reducing the number of effective data bits during high efficiency encoding of video or audio signals.
2. Description of the Prior Art
In general, the data of a video or audio signal is redundant and thus, can be trimmed by high efficiency coding for optimum transmission and recording. The high efficiency coding is commonly carried out using an orthogonal transformation method or a predictive coding technique. The former is performed by the data of an digital input signal being divided into blocks corresponding to groups of a given number of pixels and processed by orthogonal transformation in each block before orthogonally transformed data of each block is encoded. The latter is conducted by the information of a pixel to be transmitted being estimated from its adjacent pixel data and a resultant estimated error being transmitted. In such high efficiency coding procedures, the orthogonally transformed data and the estimated error are quantized and a resultant quantized value is then encoded by a fixed or variable length encoding technique for transmission and recording.
It is known that the quantization of the orthogonally transformed data and the estimated error is executed by a linear quantizing process in which the data to be quantized is divided into a given number of quantizing steps and the value of each quantizing step is converted to an integer which is given as the quantized value. In such a linear quantizing process, the division produces a decimal fraction which is in turn rounded by a data round-off device into an integer. A conventional data round-off device is arranged to perform rounding up or down by replacing the digits after the decimal point with zeros. And more familiar, the rounding up is carried out by increasing the first place digit by one if the digit in the first decimal place is 5 or greater.
However, when so-called mid tread quantization which is effective in high-efficiency-coding is used during the rounding up or down operation, considerable error of skipping one step at maximum tends to occur and a resultant code signal will exhibit degradation in the data quality. It is common that if a fractional part of the decimal fraction is 0.5 or more, it is rounded up and if not, it is rounded down. When the fractional part is 0.5, both the rounding up and down processes produce the same magnitude of a quantizing error. Usually in encoding, the smaller the absolute value of a code, the more the data can be reduced. Hence, the rounding up is not preferred when the fractional part is 0.5.